Many spacecraft and, in particular, geosynchronous communications satellites, use panels of solar cells to power the electrical equipment loads of the spacecraft. Such equipment typically include communications transponders, sensors, detectors and data processing equipment. Spacecraft typically include a battery for powering the loads when the spacecraft is out of view of the sun. Because of variations in the power consumption needs of the spacecraft, variations in the amount of power generated by the solar panels and variations in the battery condition, many spacecraft regulate the voltage and current on the spacecraft power bus which powers the electrical loads.
In one design, a battery is coupled to the spacecraft power bus through a battery charge controller and a battery discharge controller. The battery discharge controller feeds the spacecraft bus from the battery when the load is in excess of the solar panel capability. The charge controller feeds current to the battery when there is excess bus power available and the battery requires it. In other words, when there is a shortage of current on the bus, the battery is discharged and when there is a surplus of current on the bus, the battery is allowed to be recharged. Solar panels are coupled to the same bus through bus voltage limiters. The bus voltage limiters monitor the voltage of the bus and either open or close solar panel circuit connections to the bus, regulating the power supplied by the solar panels, to maintain the voltage and current on the bus in as stable a state as possible. The solar panels in such a spacecraft are designed with excess power generation capacity so that the battery charge condition can be maintained even when the other loads on the bus are heavy.
In an alternative design, it is also known to use a bidirectional battery controller to regulate the charge state of the battery. However, such a bidirectional battery controller puts additional demands on the bus voltage limiter to increase or reduce the power supply to the bus in response to the needs of the battery. The bus voltage limiter in both designs typically has a large number of circuits to control each and every circuit of the solar panel and it is a fairly heavy and complex device which adds significantly to the weight of the spacecraft. Weight significantly increases the cost of stationkeeping and of launching.
An alternative design eliminates the battery voltage limiter and the bidirectional battery controller or battery charge and discharge controllers. Instead, the power on the bus is regulated by changing the orientation of the solar panels with respect to the sun. Since a solar cell's output is a function of its angle to the sun, angling the panels controls the panels' power output. However, a typical solar panel drive requires several minutes to make any significant change in panel orientation, whereas bus loads can change in a matter of fractions of a second. Accordingly, in such a design, the voltage level on the bus varies widely and it is necessary to construct each power consuming device on the bus so that it can compensate for the wide variations in the power supply voltage. In many applications, the additional weight added to the power consuming devices compensates for any weight savings made by eliminating the battery bus voltage limiter and the battery controllers.